I would like to recommend 1p mobile as a source of mobile phone PAYG:
They've been my principal phone provider for a while now, and they're immaculate. The website is transparently easy to use, their charges are as low as they seem (i.e. you must pay around £30/year, and for that you get 3000 minutes of calls).
There doesn't appear to be any catch, they don't send spammy text messages, it's easy to turn voicemail off, and it stays off.
Highly recommended. Without doubt the best mobile phone company I've ever used, and without doubt the cheapest as well.
They have a referral link:
If you use it you and I will both get a £5 top-up.
But I do not actually care about this, I never come close to using my 3000 minutes, I'm recommending them because they're great and that sort of thing ought to be recognised and spread.